1. Field of the Invention
The present invention generally relates to an image forming apparatus, and, more particularly, it relates to an image forming apparatus wherein an image is obtained by electrostatically transferring an image electrophotographically or electrostatically formed on an image bearing member onto a transfer sheet. Such image forming apparatus is embodied as a mono-color or full-color electrophotograhic copying machine, printer or other various recording equipments.
2. Related Background Art
In the past, in an image forming apparatus such as an electrophotographic copying machine, it has been known that a toner image formed on a photosensitive drum acting as an image bearing member is transferred onto a transfer sheet at a transfer station by means of a transfer corona discharger comprising a discharging wire and a shield electrode. Further, in a full-color image forming apparatus, it has been known that a plurality of color toner images formed on the photosensitive drum are successively transferred onto a transfer sheet held on a transfer drum in a superimposed fashion by means of a transfer corona discharger.
However, as seen in the above-mentioned conventional electrophotographic copying machine, since the corona discharger was used to transfer the toner image formed on the photosensitive drum on the transfer sheet, the following problem arose.
That is to say, it is necessary to normally apply a high voltage of 6-8 KV to the discharging wire of the corona discharger, and only a portion of a corona current flows toward the photosensitive drum and others flow toward the shield electrode which is earthed. Thus, it is necessary to provide a transfer corona current greater than a current which actually contributes to the transfer operation.
Accordingly, a power source having greater voltage capacity and current capacity is required, and, insulators for the wiring must have greater pressure-resistance feature, thus making the apparatus large-sized and expensive. Further, since the corona discharger is used, the generated corona discharge creates ozone which, in tern, generates the image flow affecting a bad influence upon the photosensitive drum.
In order to solve the above problem, a method wherein, in place of the transfer corona discharger, an electrode is contacted with a back surface of a transfer sheet bearing member, i.e., dielectric sheet 93 of the transfer drum 9 to apply the voltage as shown in FIG. 10 has been proposed. According to this method, an elastic synthetic resin film 20 supported by a support 19 made of rigid material such as metal is disposed within the transfer drum 9 at a transfer station where the transfer drum 9 contacts with a photosensitive drum 3, and, further, a conductive metal sheet 21 acting as a transfer electrode is arranged on a free end of the synthetic resin film 20 and is connected to an electric power source.
Although this method could provide a simple and inexpensive device which did not generate the ozone during the transfer operation, it arose another problem. That is to say, if a paper left under a low humidity (10 %RH) condition was used as the transfer sheet, the transfer void would occur. As a result of consideration regarding this problem, the inventor has found that the transfer void occurs because the transfer electric field is generated before the photosensitive drum is contacted with the transfer sheet, thereby occurring the discharge phenomenon so that the toner is charged to a polarity opposite to a polarity charged in the developing operation. Further, this conventional method arose a problem that the toner is scattered around a portion on which an image such as a character and the like is formed later, by means of the transfer electric field generated before the contact between the photosensitive drum and the transfer sheet.